DE3932990A1 - Stone and plaster conservation - by in-situ treatment with aq. lithium poly:silicate soln. - Google Patents

Abstract

Conservation of natural stone, natural freestone and historic plasters, esp. monuments, is effected by in-situ use of an aq. lithium polysilicate soln. which is free of Na, K and polymers and which has a max. viscosity of 10 mPas, a max. solids content of 30wt.% and a Li2O/SiO2 ratio of 1:3.5-7. Pref. consolidation and conservation are carried out by infiltration, flooding or injection. ADVANTAGE - The process is environment friendly, produces no deleterious byprods., has no negative affect or physical characteristics of the stone or plaster, has good capillary mobility, produces no colour changes in the stone or plaster, and has good penetration depth in not only highly porous but also finely porous materials to give high consolidation. (5pp Dwg.No.0/0)

Description

The invention relates to a method for preservation of natural stones, natural stones and historic plasters below Use of lithium silicates.

It is known that above all historical buildings, such as architectural monuments, Sculptures or ornaments or masonry made of natural stones, and plasters by environmental influences particularly strong in Be affected. Not only the natural weathering, z. B. due to rain, snow or sun, but also the chemical and biological weathering have just recently become an essential one Increased damage caused by architectural monuments. That's why one searches for a long time for suitable preservation and Solidifying materials that have lasting effects.

From DE-OS 33 14 475 is the use of sodium, Potassium and / or lithium silicate solutions with addition of stabilizing organic polymers as binders for coating materials, such as water glass paints, water glass plasters, primers, Water glass center and water glass adhesive known; a deepening however, the treated materials are not achieved.

From DE-AS 26 52 421, the production of a mixed water glass solution of sodium / potassium with lithium silicate with viscosities  of 10-10 000 mPas known. With these solutions u. a. a concrete impregnation be made.

The known use of soda or potassium water glass as Sealing material is also disadvantageous because efflorescent Potassium carbonate (potash) or sodium carbonate (soda) on the surface of the solidified materials in crystal form is formed. Furthermore, especially with the environmental pollution produced, present at the historical plaster Calcium sulfate (gypsum) form soluble sodium or potassium sulfate and endanger the plaster. Both salts are also hygroscopic or form hydrate-water-rich forms.

The invention is based on the object, a preservation method using a preservative of the above to perform the mentioned type

• - is environmentally friendly,
• - does not lead to harmful by-products,
• - the building physics characteristics of the treated material (eg, water vapor diffusion) not adversely affected,
• - capillary is absorbed well,
• - no color changes of the treated substrate or of the material produced and
• - both in strongly porous, but also fine-pored substrates, including historical plasters, a good penetration depth owns and produces a high degree of solidification.

The object is achieved in that according to the invention a sodium, potassium and polymer-free lithium polysilicate solution with a maximum viscosity of 10 mPas, with a maximum Solids content of 30 wt .-% and a Li₂O / SiO₂ ratio  from 1: 3.5 to 7 is used in situ.

Only the extremely low viscosity of the used Lithium polysilicate solution allows penetration of the preservative z. B. in fine-pored calcite bound sandstones and even granite. A discoloration of the treated material does not occur. The agent is environmentally friendly (aqueous, solvent-free) and affects the building physics Characteristics of the treated material not negative. Since that Means purely inorganic, it also corresponds to the Claims of the historic preservation offices and restorers.

From US-PS 44 43 496 and US-PS 45 21 249 it is already known, lithium silicate solutions for impregnation and Structuring of cementitious surfaces to use. Although this achieves a silicate coating with an anchorage in the cement subsoil, but not one Conservation with deep consolidation of natural stones, whose pH value is significantly lower than that of the cement matrix. To the to obtain required permeation will be an addition of Sodium naphthalene sulfonate formaldehyde condensate proposed. The latter is dark brown in color and leads after application to an undesirable color deepening of the substrate (browning). Furthermore, this is the introduction of organic material connected. Because of today's findings of the microbial If infested with natural stones, this must be avoided otherwise the risk of microbial decomposition, associated with acid metabolic products.

According to a preferred embodiment of the invention, the Agent with a viscosity below 8 mPas, preferably below 4.0 mPas used. A particularly preferred range of Viscosity is between 3.5 and 2.0 mPas. The solids content is suitably less than 20 wt .-% and preferably between 10 and 5 wt .-%. This is particularly preferred Li₂O / SiO₂ ratio 1: 5.5 to 6.5.  

Are cavities to be filled, especially hollow plaster surfaces to fill up is preferably a 5-25% Lithium polysilicate solution to use. Procedurally so The agent was soaked, flooded or injected can be; This treatment can also be repeated several times, eg. Up to four times, be made.

example A

A heavily weathered historic, 16th century originating lime plaster with a layer thickness of 5-15 mm with a 7% aqueous lithium polysilicate solution with a Viscosity of 2.25 mPas soaked twice full; already after About 14 days became a continuous uniform solidification reached. Only by a considerable mechanical effort The plaster could be broken. There were no efflorescence (whitish coating) detectable after treatment.

example B

A heavily weathered historic, 16th century originating lime plaster was compared with a known Potassium silicate solution saturated twice. It was on the Lime plaster a crusting and a whitish coating on the Cleaning surface recognizable. The solidification took off from the surface towards the inside of the plaster.

example C

After another comparison was a heavily weathered historical lime plaster from the 16th century with a known, commercially available silicic acid ester twice soaked. Less solidification was achieved as in the treatment with the known potash waterglass solution after example B. Here too it could be stated that the Solidification of the plaster surface towards the inside decreased. The plaster was easily crumbled even after the treatment.  

example D

A heavily weathered granite stone with a layer thickness of 6 cm was relatively easily crushed by rubbing with a finger become. This granite was with a 10% aqueous lithium polysilicate solution having a viscosity of 2.7 mPas and a Li₂O / SiO₂ ratio of 1: 4.6 halves twice soaked. Already after about 14 days qualitatively decisive Strength differences are found. The of the granite surface treated to the air side was down to the bottom evenly consolidated and could only under high mechanical Costs are slightly destroyed from the edge.

The effectiveness of stone conservation or stone strengthening agents is by checking the features

• biaxial flexural strength,
• - reduction of the pore space,
• - reduction of water absorption,
• - influence on the hygric change in length and
• - Change of the water vapor diffusion behavior

detected.

An essential feature of the effectiveness of the solidifying agent is the capillary absorption capacity.

The results listed below are among the experts been determined by known examination methods.

Example E 1. Rüthener sandstone: Zero value 3.9 N / mm² After treatment with a 20% strength lithium polysilicate solution having a viscosity of 4.5 mPas and a Li₂O / SiO ratio of 1: 5.9 9.0 N / mm² After treatment with a 10% lithium polysilicate solution having a viscosity of 2.7 mPas and a Li₂O / SiO₂ ratio of 1: 6.0 7.4 N / mm² 2. Sander sandstone: @ Zero value 5.0 N / mm² After treatment with a 10% lithium polysilicate solution having a viscosity of 2.7 mPas and a Li₂O / SiO ratio of 1: 5.8 8.0 N / mm² After treatment with a 5% lithium polysilicate solution having a viscosity of 2.25 mPas and a Li₂O / SiO₂ ratio of 1: 6.0 7.0 N / mm²

example F

To test the capillary rise height was on a drill core with a height of 10 cm at the Rüthener sandstone a height detected by 10 cm, d. H. the stone was completely capillary soaked. This was proven by spraying the drill core with a 0.1% phenolphthalein solution. The lithium polysilicate solution possessed a pH of about 9.8-10.0 (each by degree of dilution) and therefore led to red violet staining the phenolphthalein solution. The Li₂O / SiO₂ ratio was 1: 6.1 at a solids content of 21%.

example G

To test the reduction of the pore space was as follows The procedure:

1. Rüthener sandstone: Zero value 16 vol.% After treatment with a 20% lithium polysilicate solution having a viscosity of 7 mPas and a Li₂O / SiO₂ ratio of 1: 4.2 13 vol.% After treatment with a 10% lithium polysilicate solution having a viscosity of 2.7 mPas and a Li₂O / SiO₂ ratio of 1: 6 13 vol.% After treatment with a 5% lithium polysilicate solution having a viscosity of 2.25 mPas and a Li₂O / SiO₂ ratio of 1: 5.9 14.8% by volume 2. Very fine-pored Baumberger sandstone: @ Zero value 15 vol.% After treatment with a 5% lithium polysilicate solution having a viscosity of 2.2 mPas and a Li₂O / SiO₂ ratio. from 1: 6 13 vol.%

The water vapor diffusion resistance coefficient became after determined two methods, namely the dry-cup method and after the wet-cup method.

The following values were determined by both methods:

dry-cup method Rüthener sandstone: Zero value μ = 25 After treatment with a 10% lithium polysilicate solution having a viscosity of 2.7 mPas and a Li₂O / SiO₂ ratio of 1: 5.9 μ = 17 Sander Sandstone: @ Zero value μ = 35 Treated with a 5% lithium polysilicate solution having a viscosity of 2.1 mPas and a Li₂O / SiO₂ ratio of 1: 5.9 μ = 28 Baumberger Sandstone: @ Zero value μ = 31 Treated with a 5% lithium polysilicate solution having a viscosity of 2.1 mPas and a Li₂O / SiO₂ ratio of 1: 5.9 μ = 38

wet-cup method Rüthener sandstone: Zero value μ = 19 After treatment with a 10% lithium polysilicate solution having a viscosity of 2.7 mPas and a Li₂O / SiO₂ ratio of 1: 5.9 μ = 17 Sander Sandstone: @ Zero value μ = 25 After treatment with a 5% lithium polysilicate solution having a viscosity of 2.1 mPas and a Li₂O / SiO₂ ratio of 1: 5.9 μ = 23 Baumberger Sandstone: @ Zero value μ = 24 After treatment with a 5% lithium polysilicate solution having a viscosity of 2.1 mPas and a Li₂O / SiO₂ ratio of 1: 5.9 μ = 20

The influence on the water vapor diffusion ability is of crucial for the behavior of damaged sandstones. Since the porosity, d. H. the pore geometry and the Radiation distribution in natural stones is not reproducible, can only by a variety of experiments the result statistically be secured. Much easier and with much less Expenditure is connected to the test on defined glass frits, d. H. Glass frits of defined porosity. Again, it was here again the water vapor diffusion or the water vapor diffusion coefficient μ determined by the dry-cup and the wet-cup method.

Below values, determined on glass frits D 4:  

dry-cup method Zero value μ = 30 After treatment with a 10% lithium polysilicate solution having a viscosity of 2.7 mPas and a Li₂O / SiO₂ ratio of 1: 5.9 μ = 26 After treatment with a 5% lithium polysilicate solution having a viscosity of 2.05 mPas and a Li₂O / SiO ratio of 1: 6 μ = 23

wet-cup method Zero value μ = 22 After treatment with a 10% lithium polysilicate solution having a viscosity of 2.7 mPas and a Li₂O / SiO₂ ratio of 1: 5.9 μ = 18 After treatment with a 5% lithium polysilicate solution having a viscosity of 2.05 mPas and a Li₂O / SiO₂ ratio of 1: 6 μ = 15

Reduction of water absorption

By reducing the pore space is also a reduction to expect the water absorption behavior. For reasons of reproducibility were again glass frits with a defined Pore geometry (designation D 4) used.

Below are the values:

Zero value 44.6% by volume After treatment with a 10% lithium polysilicate solution having a viscosity of 2.7 mPas and a Li₂O / SiO₂ ratio of 1: 6 35.3% by volume After treatment with a 5% lithium polysilicate solution having a viscosity of 2.1 mPas and a Li₂O / SiO₂ ratio of 1: 5.9 40.9% by volume

binder growth

To determine to what extent the capillary absorbency too a binder increase increase has led, are suitable Both coarse to medium-pored sandstones, but rather again  the glass frit quality described above.

The following values were obtained with glass frit quality D 4 determined:

After treatment in a 5% lithium polysilicate solution - binder addition: | 3.9% After treatment in a 10% lithium polysilicate solution - binder addition: 6.9%

Li₂O / SiO₂ ratio in both cases: 1: 5.8.

Claims (8)

1. A method for the preservation of natural stones, natural stones and historic plasters, in particular monuments using lithium silicates, characterized in that an aqueous, sodium, potassium and polymer-free lithium polysilicate solution having a maximum viscosity of 10 mPas, a maximum solids content of 30 wt. -% and a Li₂O / SiO₂ ratio of 1: 3.5 to 1: 7 is used in situ. 2. The method according to claim 1, characterized in that a aqueous lithium polysilicate solution having a viscosity below 8 mPas, preferably below 4 mPas. 3. The method according to claim 2, characterized in that a Lithium polysilicate solution having a viscosity of 3.5 to 2.0 mPas is used.   4. The method according to one or more of claims 1 to 3, characterized in that an aqueous lithium polysilicate solution with a solids content below 20% by weight, preferably 10 to 5 wt .-%, is used. 5. The method according to one or more of claims 1 to 4, characterized in that an aqueous lithium polysilicate solution is used, whose Li₂O / SiO₂ ratio 1: 5.5 to 1: 6.5. 6. The method according to one or more of claims 1 to 5 for Filling cavities, in particular hollow plaster surfaces, characterized in that an aqueous lithium polysilicate solution with a solids content of 5 to 25% by weight is used. 7. Method for preserving natural stones, natural stones and historic plasters, especially historical monuments, according to one or more of claims 1 to 6, characterized characterized in that the solidification and preservation by soaking, flooding or injecting. 8. The method according to claim 7, characterized in that the Treatment is done several times.